1. Field of the Invention
This invention relates to a cooling method and a cooling mold for continuous casting of ingots from molten aluminum, aluminum alloys, or other metals and more particularly to a method of continuous and direct chill casting and a mold for carrying out the direct chill casting method.
2. Description of the Prior Art
In this continuous casting method as shown generally in FIG. 7, a molten metal 13 is injected from a tundish 11 through an orifice plate 15 into a mold 12 which is water-cooled, so that the molten metal 13 is cooled in the mold 12 to cast an ingot 14. The molten metal 13 which is introduced through the orifice plate 15 to the mold 12 is contacted with the wall surface of the mold 12 to form a thin solidified shell and is further cooled and cast with impinging cooling water applied from the mold 12.
In continuous casting, a higher rate of casting is desired to improve the production rate and in order to realize the higher rate of casting, it should be simultaneously required to promote the casting quality such as the surface condition of the ingot by proper cooling.
In high rate casting, when the molten metal is solidified in the cooling mold to form the solid shell, the higher rate of casting requires the greater amount of heat extraction and thereby the larger amount of cooling water. The cooling water is applied from the mold to directly impinge on the high temperature ingot and cool it. However, when the casting rate is increased, since the surface temperature of the ingot becomes higher in a situation of impingement cooling with cooling water, a transition boiling zone and a film boiling zone is produced on the ingot surface and a vapor film which creates an adiabatic phase between the ingot surface and the cooling water is formed thereon. Thus, even if the amount of the cooling water is increased, the cooling water does not effectively function to carry out heat extraction so that the danger of break out increases, and problems such as causing quality defects of the ingot arise. Hence, these problems have been the factors which have considerably reduced the casting stability and the quality stability.
In order to solve these problems, cooling methods have been proposed in which directly impinging cooling water is used in two steps as disclosed for example in Japanese Patent Laid-Open Application 58-212849.
However, in the two step cooling method using the cooling water as disclosed in the above Japanese Patent publication, since the distance between the first cooling zone and the second cooling zone becomes considerably long, that is one-half to two times the diameter of the ingot, the surface of the ingot which has been cooled in the first cooling zone is again heated by the time it reaches the second cooling zone due to heat flow from an internal region of the ingot. Hence, even when a second cooling is carried out, the transition boiling and film boiling phenomena are again produced reducing cooling efficiency. When using high rate casting, this tendency is even more increased which considerably reduces the cooling efficiency.